Abstract

Small-angle neutron scattering (SANS) has been used to study the adsorption behavior of supercritical carbon dioxide in porous Vycor glass and silica aerogels. Measurements were performed along two isotherms ( and ) as a function of pressure ranging from atmospheric up to , which corresponds to the bulk fluid densities ranging from . The intensity of scattering from -saturated Vycor porous glass can be described by a two-phase model which suggests that does not adsorb on the pore walls and fills the pore space uniformly. In -saturated aerogels an adsorbed phase is formed with a density substantially higher that of the bulk fluid, and neutron transmission data were used to monitor the excess adsorption at different pressures. The results indicate that adsorption of is significantly stronger in aerogels than in activated carbons, zeolites, and xerogels due to the extremely high porosity and optimum pore size of these materials. SANS data revealed the existence of a compressed adsorbed phase with the average density , close to the density corresponding to closely packed van der Waals volume of . A three-phase model [W. L. Wu, Polymer23, 1907 (1982)] was used to estimate the volume fraction of the adsorbed phase as a function of the fluid density, and gave in the maximum adsorption regime around . The results presented in this work demonstrate the utility of SANS combined with the transmission measurements to study the adsorption of supercritical fluids in porous materials.

Received 08 August 2005Accepted 11 April 2006Published online 26 May 2006

Acknowledgments:

We gratefully acknowledge P. E. Wolf for careful reading the manuscript, stimulating discussions, and sharing with us the data on confined prior to publication. This research was sponsored by the Laboratory Directed Research and Development Program and by the Divisions of Materials Sciences and Engineering and Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences, U.S. Department of Energy under Contract No. DE-AC05-00OR22725, Oak Ridge National Laboratory, managed by UT-Battelle, LLC.